 \begin{tikzpicture}[
node distance = 2cm and 2cm,
     N/.style = {circle, draw, font=\Large, 
                 minimum size=1.1em},
       every edge/.style = {draw, -Stealth, semithick, shorten >=2pt},
every edge quotes/.style = {auto=right, inner sep=2pt}
                        ]

\node[N,align=center] (a1) at (0,0) [label=left :{+ -}] {1};
\node[N] (b1)[right of=a1] {2};
\node[N] (c1)[right of=b1][label=right :{-}]{3};
\node[N] (d1)[below of=b1]{4};
\node[N] (e1) [right of=d1][label=right :{-}]{5};
\path
(a1) edge [above] node {a} (b1)
    edge [above] node {a} (d1)
(b1) edge [above] node {c} (c1)
(b1) edge [loop above] node {b} ()
(d1) edge [above] node {b} (e1)
(e1) edge [bend right] node [above] {a} (d1)
    ;
    \end{tikzpicture}